I’m not aware of any metro area that’s building affordable high rise housing. The cost of construction goes up exponentially as you increase a building’s height. Even if SF had no restrictions on height I doubt most construction would be over 10 stories due to fire/building codes getting much more restrictive. So you’re left with a developer paying ~$3M to acquire a duplex that they can then build a max of 10 units on. Just the cost of the land is going to require $300K from each unit sold. Add in the construction costs, and a modest profit and you’re back up to 1M+ unit cost. 1M condo is still not affordable for working class residents.

It's possible to build on multiple brownstones and replace it with a denser multifamily but:

  You have to own the lots

  You have to get permits and go through a lot of reviews by the city and possibly the state (if it's condos)

  You'll still be faced with aging infrastructure.  The subway is 2/3 capacity at most and still there are delays and crowded train cars.

I'm not questioning the wisdom of higher density, I'm questioning the feasibility of it. How many years and how much did it cost to make a few blocks of 2nd avenue subway?

London and Paris appear to be doing a better job at expanding the city outward, adding infrastructure as they go.

There is something deeply, fundamentally wrong with modern urban growth in the U.S., and I suspect it has to do with the decentralized nature of the political system. How many layers of approvals do you need here and how much coordination do you need with how many agencies across how many levels of government from local to Federal.